Liquid crystal display devices (LCD) typically include liquid crystal cells, with two polarizing plates positioned on either side of the cells. In a reflective liquid crystal display device, a reflecting plate, a liquid crystal cell, and a single polarizing plate are laminated together. In these liquid crystal display devices, in order to increase the viewing angle, eliminate coloring, or adjust the phase difference in accordance with the display mode, an optical compensation sheet (a retardation plate) formed from a type of optically anisotropic medium is often positioned between the liquid crystal cell and the polarizing plate.
Typically, a polymer film with birefringence is used in the optical compensation sheet, and for example, a polymer film with birefringence, formed from an optically anisotropic medium that is produced by subjecting a polymerizable liquid crystal material to alignment treatment and subsequently curing the material with ultraviolet radiation to fix the alignment state, is currently in use as an LCD optical compensation sheet.
Furthermore, recent years have seen much progress in the development of liquid crystal device production methods such as Roll to Roll methods, which have introduced a coating process with the aim of dramatically improving the production efficiency compared with conventional batch production methods. The optical compensation sheet for a Roll to Roll application is prepared using a coating method, and is designed with the premise that another member will be laminated on the produced sheet.
In those cases where an optically anisotropic medium is prepared using a polymerizable liquid crystal material, the polymerizable liquid crystal material is usually sandwiched between two alignment films, and the alignment restraining forces from both sides are then used to align the liquid crystal molecules. However, in those cases where a coating process such as a Roll to Roll process is used in the production of an optical compensation sheet, or in cases where the process is shortened, the polymerizable liquid crystal material is applied to the surface of a substrate that exhibits an alignment function, and the alignment of the polymerizable liquid crystal material is then conducted using only the alignment restraining force from the alignment film on one side of the material. In other words, unlike the former case, the alignment restraining force tends to act more weakly on the liquid crystal molecules in the vicinity of the interface between the material and the air, which is not in contact with an alignment film. Accordingly, defects are generated at the air-liquid interface, and the quality or yield of the obtained optically anisotropic medium tends to decrease.
In order to resolve this problem, additives have been proposed that are mixed with the polymerizable liquid crystal material and control the alignment of the liquid crystal molecules in the vicinity of the air interface, and known examples of these additives include liquid crystal alignment promoting agents formed from compounds that include, within each molecule, a hydrophobic group such as a fluorine-substituted aliphatic group or an oligosiloxane group, and a group that contains at least two ring structures and exhibits an excluded volume effect (for example, see patent reference 1).
Because the hydrophobic groups of the above compounds exhibit poor co-solubility with the liquid crystal molecules, they tend to be mostly distributed along the interface with the air. In contrast, the groups within the compounds that exhibit an excluded volume effect are soluble with the liquid crystal molecules, and consequently become embedded within the liquid crystal layer. By adopting a combination of a hydrophobic group and a group that exhibits an excluded volume effect, the tilt angle of the liquid crystal molecules at the air interface can be controlled arbitrarily, regardless of the nature of the liquid crystal molecules.
As a result, an optically anisotropic medium with superior alignment can be obtained.
However, with the compounds described above, alignment problems at the air-liquid interface have been unable to be completely suppressed. Furthermore, because the above compounds are low molecular weight compounds, it is impossible to ensure that all the molecules are distributed along the air interface, and a portion of the molecules remains within the bulk of the liquid crystal layer, which can cause a reduction in the phase transition point of the liquid crystal material, and increases the danger of a reduction in the stability of the produced optically anisotropic medium and a decrease in retardation.
Furthermore, examples of known optically anisotropic media produced by curing a mixture of a polymerizable liquid crystal material and a polymer compound include a homeotropically aligned liquid crystalline composition containing a side-chain liquid crystal polymer having alkyl groups of 1 to 22 carbon atoms or fluoroalkyl groups of 1 to 22 carbon atoms and a photopolymerizable liquid crystal compound, which is capable of forming a homeotropically aligned liquid crystal layer on top of a substrate provided with no vertical alignment film, as well as a homeotropically aligned liquid crystal film produced by light irradiation of the aligned state (for example, see patent reference 2).
The above side-chain liquid crystal polymer exhibits homeotropic alignment, and by mixing in at least 10% of this polymer, a homeotropically aligned liquid crystal film with excellent durability is obtained. However in this method, an optically anisotropic medium with a tilted alignment or horizontal alignment cannot be produced. Furthermore, in the above publication, only polymers having alkyl groups of 1 to 22 carbon atoms are disclosed specifically as the side-chain liquid crystal polymer, and no specific disclosure is made of side-chain liquid crystal polymers having fluoroalkyl groups of 1 to 22 carbon atoms. Accordingly, absolutely no suggestions are made as to what types of properties or effects side-chain liquid crystal polymers having fluoroalkyl groups may possess.
Furthermore, in another known method of forming a thin film, a surfactant material that reduces the intrinsic tilted alignment of the director of the liquid crystal material at the air interface is added to the polymerizable liquid crystal material, thereby causing the alignment of the liquid crystal material at the air interface of the liquid crystal alignment layer to adopt an essentially parallel or essentially tilted configuration (for example, see patent reference 3). Specific examples of preferred surfactant materials include polyacrylate esters, polysilicon, reactive polysilicon, organosilanes, waxes, and release agents, and by using a polycyclohexyl methacrylate or a polymethyl methacrylate, the tilt angle through the thickness direction can be altered. However, the surfactant materials disclosed in this publication are unable to completely resolve the problem of defects in the vicinity of the surface.
[Patent Reference 1]
Japanese Unexamined Patent Application, First Publication No. 2000-345164
[Patent Reference 2]
Japanese Unexamined Patent Application, First Publication No. 2003-227935
[Patent Reference 3]
Japanese Unexamined Patent Application, First Publication No. 2000-105315